The present invention relates to a rare earth-iron-based permanent magnet which includes a rare earth element, boron, and iron as its principal constituents.
In the past, a rare earth-Co-based magnet is known as a high performance magnet. Since, however, the maximum energy product (BH).sub.max of the rare earth-Co-based magnet is not large enough, being about 30 mGOe at most, the strong demand in recent years for smaller size and higher performance in electronic apparatus, makes it desirable to develop a permanent magnet with higher performance. In response to such a demand, development has been ongoing for a permanent magnet that has iron as its principal constituent (See, for example, European Patent Application 101552, U.S. Pat. Nos. 4,402,770, 4533408, 4541877, and others). The iron-based permanent magnet includes a rare-earth element (R) such as Nd, and boron (B) with the balance occupied essentially by iron (Fe). It makes use of Fe which is less expensive than Co as the principal ingredient, and is capable of producing (BH).sub.max that can exceed 30 mGOe. Therefore, it represents an extremely promising material that can provide a high performance magnet at low cost.
The drawback of the iron-based permanent magnet is that the Curie temperature (Tc) is low compared with the permanent magnet of rare earth-Co-based permanent magnet, and has an inferior temperature characteristics of the magnetic characteristics. This will become a serious problem when it is to be used for a DC brushless motor or the like that is operated under conditions such as high temperature environment, and hence an improvement on this aspect has been desired.
As such an improvement, there has been proposed a composition such as R--B--Co--Al--Fe (EPA No. 106948). The present inventors have also filed applications, i.e., application 773,547, filed in 1985 and application 843,286, filed in 1986. However, the addition of Co brings about a deterioration in the magnetic characteristics so that a demand is strong for improving the performance of the rare earth-Fe-based permanent magnet, with development efforts under way at various laboratories.